In the following discussion of the Figures, the first digit of a reference numeral to an element in a figure indicates the first figure in which that element is presented.
This invention relates in general to instruments for measuring the properties of optical fibers and relates more particularly to optical time domain reflectometers (OTDRs). In the OTDR, a laser provides pulses of light that are injected into an optical fiber under test to measure the impulse return function (i.e., the return signal for a single pulse) of the optical fiber. An optical detector converts the return signal into an electrical signal that is amplified, sampled at a succession of time t.sub.i, and then converted by an analog-to-digital converter (ADC) to digital data.
An OTDR tests an optical fiber by launching one or more test pulses of light into an input end of the fiber and measuring the return signal produced by these test pulses at the input end of the fiber. Each pulse produces an impulse return function h(t), representing the magnitude of the energy incident on the detector as a function of time, having the general shape shown in FIG. 1. h(t) is an exponentially decreasing function that also includes some step drops, like step drops 11 and 12 in amplitude and may include some spikes like spike 13.
As a test pulse travels down the optical fiber, Rayleigh scattering produces an exponentially decreasing amplitude of the pulse. Some of this scattered light reaches the input end of the optical fiber to produce the measured return signal. It is this exponentially decreasing amplitude that produces the exponentially decreasing shape of the return function.
Discrete scattering centers produce step decreases in the amplitude of the test pulse that show up as step drops in the return signal. An important example of a discrete scattering center is a splice at which two optical fibers are joined end to end. The loss of energy by the test pulse can result from misalignment of the two ends that are joined at the splice and can also result from a difference in diameter of the two fibers. A scattering spike results if a discrete scattering center scatters a significant fraction of the test pulse back to the input end of the fiber. Depending on how the output end of the fiber is terminated, Fresnel scattering at the output end can produce a large spike at the end of the return signal. The locations of these discrete scattering centers is often of interest and determines which sections of the optical fiber are of particular interest when testing the fiber. A continuous section of the fiber that is of interest is referred to herein as a "window".
Because the return signal can be quite small, there exist OTDRs that inject a plurality of test pulses to produce the return signal (for example, see P. Healey, "Optical Orthogonal Pulse Compression by Hopping", Electronics Letters 17, 970-971; or P. Healey, "Pulse Compression Coding in Optical Time Domain Reflectometry", 7ECOC, Copenhagen, Denmark, September, 1981; or copending U.S. patent application Ser. No. 935,661 entitled "Spread Spectrum Optical Time Domain Reflectometer" filed by Moshe Nazarathy, et al on Nov. 26, 1986. These test pulses each produce an associated return signal. When these test pulses are closely spaced, their associated return signals will overlap as illustrated in FIG. 2B for a test signal defined by an eight bit Golay code (shown in FIG. 2A).
The impulse return function in FIG. 1 has a decay time on the order of milliseconds and the pulses in FIG. 2 have a width on the order of microseconds. Thus, on the time scale in FIG. 2, the portion of h(t) in FIG. 2B produced by a corresponding pulse in FIG. 2A is substantially constant over the time scale in FIG. 2. In FIG. 2A, the eight bit code has six bits (bits 21-23, 25, 26 and 28) of unit amplitude and 2 bits (bits 24 and 27) of zero amplitude. Thus, the measured return signal x(t) represented by curve 210', is a superposition of impulse return functions produced by each of the nonzero bits in FIG. 2A. Thus, pulses 21-23, 25, 26 and 28 produce the overlapping impulse return functions 21'-23', 25', 26' and 28'. These combine to produce the measured return function x(t). At point 29, the buildup is complete and the general exponential decay becomes apparent.
At the end of each set of test pulses is a "dead time" in which no further test pulses are injected into the fiber. This dead time allows the return signals for one set of test pulses to end before a subsequent return signal is generated by the next set of test pulses. Each set of test pulses plus its subsequent dead time is referred to as a "shot" and the measured return function produced by a shot is referred to as the "shot return function". Various patterns of test pulses are utilized to enable the impulse return signal h(t) to be extracted from the measured return signal x(t). In the copending patent application by Nazarathy, et al, cited above, at least two types of shots need to be transmitted--each type is encoded according to one of the codes in a Golay pair. Each of these types of shots produces an associated shot return function. By measuring both types of return functions, the impulse return function h(t) can be extracted from both types of x(t).
Unfortunately, the amplitude of the shot return signal can exceed the dynamic range of the OTDR. Therefore, a method of selecting new OTDR operating parameters and splicing together the partial results is needed that enables the production of an output display of data within a time period that is acceptable to a typical OTDR user.